Inorganic-organic polymer composite materials are used in a wide variety of applications including structural materials, high performance composites, optical components, aerospace, biomedical implants and dental applications. Generally, composites are employed where performance requirements are demanding and not easily fulfilled with traditional structural materials. For example, inorganic materials such as glass, ceramic and stone are very hard, scratch resistant and even sometimes transparent (e.g., glass) but suffer from the fact that they are very heavy and brittle. Polymers, conversely, are light and durable but have poor hardness, abrasion and wear resistance. Composites, made from the combination of inorganic materials and polymers, may have properties that lie in between, potentially providing materials that are simultaneously strong but lightweight, hard but flexible, abrasion resistant and durable.
In order to achieve such properties, in practice, hard inorganic materials are mixed into polymers, or polymer precursors, monomers and/or oligomers, (hereafter collectively referred to as resins) and the mixture is then cured to form a composite. In recent years, the polymer industry is transforming from composites that are polymerized, or “cured”, using heat (thermal set polymers) to composites that are cured using ultraviolet or visible light, or low energy electrons (hereafter called UVEB resins). UVEB curable resins offer tremendous energy and waste savings to the coatings and composites industries because they are polymerized (cured) directly with light or low energy electrons, and also because they generally do not contain volatile diluents such as solvents or carriers that may be considered hazardous air pollutants. UVEB curing is far more energy efficient since it overcomes the thermal loss that is prevalent in conventional thermoset coating systems. Ironically, the fundamental advantages of UVEB systems, where a solventless medium is cured rapidly by radiation, are also the source of significant system limitations.
Light curing requires that the coating and/or object must be sufficiently transparent in the spectral region of curing, since the penetration depth and absorption of the curing radiation is essential to achieve rapid and efficient curing. This limits the performance additives (fillers, stabilizers, functional additives, and coating aids) that can be added to UVEB systems since the additives must also fulfill the requirement of being sufficiently transparent in the curing region of the spectrum. Furthermore, in thick coatings or composites, the degree of curing may vary across the specimen due to the attenuation and absorption of curing radiation. To overcome this problem, in practice, it is common to “overexpose” the specimen with curing radiation in order to assure that curing is complete or near complete. This is not ideal since energy and time are thereby wasted. Furthermore, in medical applications such as dentistry, overexposure may increase risk to the patient.
The dental industry, primarily due to health concerns, is rapidly transitioning dental restoratives (e.g., cavity fillings, dental restorations, adhesives, etc.) from the conventional mercury based amalgams to highly filled, light curable, resin based composites. Resin based composites are safer and better match the color and appearance of human tooth enamel, but are often softer, not as strong or as durable as the traditional metal amalgams. To resolve these problems, manufacturers have developed microfilled polymer composites that have strength, hardness and durability close to that of the conventional amalgams. Typically the resin based composite paste is applied or packed into a tooth cavity and then cured using a hand-held light wand. The light wand is held in proximity of the composite for a period of time believed necessary to fully cure the paste with the intention to create a hard, strong and durable composite.
There is a significant clinical problem, however, in that inadequate curing can lead to premature failure of the composite requiring clinical revision of the restoration and significant patient cost. The extent and significance of the problem has been described in recent dental publications including “Light-Curing Units: A Review of What We Need to Know”, Price et al., Journ. Dental Res. (2015), and “Light-curing of resin based composites in the LED era”, Kramer et al., American Journ. Dentistry (2008), and are incorporated herein by reference. The cure rate and cure depth of a restoration is dependent upon a number of factors including the composite thickness, composite color, light absorption and attenuation of light within the composite. This is further compounded by the variability in lamp designs and power outputs of lamps from various manufacturers, and the degradation of the lamp over time, and yet even further complicated by user variability in terms of how far the lamp tip is held from the composite and for how long the composite is irradiated with polymerizing light. Today, a dentist may follow manufacturer's guidelines, but still has no method of determining if the restoration was sufficiently cured.
PCT WO 2011/140469 to Fathi et al., discloses a polymerizable composition including at least one monomer, a photoinitiator capable of initiating polymerization of the monomer when exposed to light, and a phosphor capable of producing light when exposed to radiation (typically X-rays). The material is particularly suitable for bonding components at ambient temperature in situations where the bond joint is not accessible to an external light source. There is a problem, however, in that the invention is directed toward curing (with X-rays) opaque structures that are not accessible to UV or visible light. There is an additional problem in that the invention does not include a detector system capable of indicating that polymerization is substantially complete.
U.S. Pat. No. 9,211,695 B2 to Paulson is directed toward monitoring the polymerization of photopolymerizable inks and discloses a monitoring device including a light source, an optical filter, and an optical detector. The monitoring device may monitor curing processes, such as ultraviolet (UV) curing processes to determine the progression of the level of cure of a light-activated material to a substrate. The infrared light source emits light toward a light-activated material, such as a film, and/or a substrate. The optical filter is positioned so that a wavelength of the light is transmitted through the optical filter after the light is reflected off of the substrate and/or the film. The optical detector is positioned to detect the light that is transmitted through the optical filter. There is a problem however in that this method is directed toward measuring the change in reflectance of a photopolymerizable target after it has been polymerized, it requires multiple light sources and is optically complex, and further the method is not demonstrated by working examples in the patent.
U.S. Pat. No. 7,553,670 B2 to Rakow et al. discloses a method of monitoring a polymerization in a three dimensional sample comprising an initiation surface and a separate one or more monitoring surfaces, said monitoring surfaces substantially perpendicular to said initiation surface, wherein the method comprises initiating said polymerization at or on said initiation surface, and capturing a thermographic profile at a plurality of points on at least one monitoring surface of said sample with an infrared detector array. There is a problem however in that the method requires a perpendicular geometry, it requires bulky infrared cameras, and cannot be easily adapted to a hand-held device. There is a further problem in that the method does not provide for automation of the device.